Generally, in a manufacturing method of cathode ray tubes, a knocking treatment is performed in the post process of the manufacturing assembly processes, as a process for improving the withstand voltage characteristics, to remove fine projections on the electrodes constituting an electron gun, flashes and fluffs generated by the pressing formation, and dust and other foreign substances affixed to the electrode surfaces in order to reduce current leakage and intensify the resistance against heat and vibration from outside for the purpose of obtaining a long and stable performance.
Conventionally, as knocking treatment process of the kind, a spot knocking method, an indirect knocking method and others are known.
FIG. 6 is a view showing the connecting structure of an example in which a conventional spot knocking method is applied to a large color cathode ray tube (29", 31", etc.) with an electron gun of EA-DF type (elliptical aperture dynamic focus type with elliptical electron beam passage). FIG. 8 is a view showing the actual structure of the EA-DF type electron gun as a reference. In FIG. 6 and FIG. 8, a reference mark G1 designates a first grid electrode; G2, a second grid electrode (screening electrode); G3, a third grid electrode (controlling electrode); G4, a fourth grid electrode; G5, a fifth grid electrode (in practice, the electron gun comprises each unit of G5-1, G5-21, and G5-2); G6, a sixth grid electrode (anode unit); SC, a shield cup; K, a cathode electrode; and ST, a stem. Between the G2 electrode and G4 electrode, and the G3 electrode and G5 electrode, the inner connections are respectively made. In this respect, the cathode K should be grounded when the knocking is performed, but such grounding is not shown in FIG. 6 in order to simplify the representation.
The usual operating voltages are -60 V to 0 V for the cathode K, 0 V for the G1 electrode, 600 V for the G2 and G4 electrode, 9 KV (applied as a focus voltage Vf of approximately 28% of Eb) for G3 and G5 electrodes, and approximately 30 KV (high voltage supply Eb) for G6 electrode, and an electron lens focusing system is constituted between G3-G4, G4-G5, and G5-G6.
In the spot knocking method, the cathode electrode K, G1 electrode, G4 and G2 electrodes and G5 and G3 electrodes are all grounded as shown in FIG. 6, while only the G6 electrode is connected to a high-voltage supply Eb having an induction two times the operating voltage (for example, a positive pulse voltage of 70 KV, 50 Hz, pulse width 0.05 ms, for example). Thus, the knocking treatment is performed by generating a spark from the G6 electrode to the G2 electrode through the G5 or G3 electrode.
FIG. 7 is a view showing the connecting structure of an example by the conventional technique, in which an indirect knocking method is applied to a color cathode ray tube also with the EA-DF type electron gun. In this knocking method, what differs from the structure shown in FIG. 6 is that the G5 and G3 electrodes are grounded through a resistor R2 (10 K.OMEGA.) instead of the direct grounding thereof. In this indirect knocking method, a spark current flows into the resistor R2 by the sparking from the G6 electrode to the G5 and G3 electrodes. Hence, a voltage VG3 is induced to the electrode G5 and G3, and this induced voltage VG3 causes a secondary spark from the G5 and G3 electrodes to the G4 and G2 electrodes to perform the knocking. Thus, this knocking is called the G2-G3 indirect knocking.
In this respect, there is known a float knocking method (refer to Japanese Patent Laid-Open No. 154034/1980), wherein the G5 and G3 electrodes in the spot knocking method shown in FIG. 6 are opened instead of being grounded, and a high voltage is applied to the G6 electrode.
The spot knocking method, G2-G3 indirect knocking method or the float knocking method by the above-mentioned conventional technique are all such that the high voltage for knocking, which is supplied from the outside, is applied only through the G6 electrode. Therefore, the sparking is easily generated from the G6 electrode to the comparatively high electrodes (nearer to the G6 electrode), but it is always difficult to generate the sparking to the comparatively low electrodes (farther from the G6 electrode). Consequently, there is the problem that the A stray (a cold emission from the said electrode reaches the phosphor screen to illuminate it) by the lower electrodes and the B stray (a leak current by emission between the electrodes) are generated.
Particularly, in the above-mentioned electron gun, such as the EA-DF type and the EA-UB type (elliptical aperture unipotential-bipotential type), for which the focusing has been improved recently to obtain a better quality of the larger color cathode ray tubes of 29", 30" or the like, the distance between the G6 electrode and the G2 electrode is long as compared with the conventional B-U type (bipotential-unipotential type) electron gun. Therefore, it is easy to effectuate the knocking by the comparatively high controlling electrode G3 (nearer to the G6 electrode) but it is difficult to effectuate the knocking by the low G2 electrode farther from the G6 electrode. Hence, there is the problem that the G2 A stray (A stray by the emission of the G2 electrode) defect occurs quite often in the course of the manufacturing process.
Also, a higher voltage may be applied to the G6 electrode in order to make it easier to effectuate the knocking by the low electrodes (farther from the G6 electrode). However, the voltage to be applied to the anode electrode is limited to a certain level because it can cause dielectric breakdown or creeping discharge between the lead wires or terminals.
Therefore, an object of the present invention is to provide a manufacturing method of cathode ray tubes, wherein a relatively low voltage is applied to knock lower electrodes such as G2 so as to eliminate defects such as G2 A stray) while solving the above-mentioned problems encountered in the conventional technique.